Both ultrastructural and biochemical data accumulated thus far indicate that sialyltransferase, but not other glycosyltransferases, exists at the surface of L-1210 cells. The enzyme is not reactive with exogenous acceptor molecules and is only capable of glycosylating acceptor molecules on the same cell surface. The acceptor molecules are membrane glycoproteins which may be capped and internalized by the cell following treatment with con A and colchicine. L-1210 cells are unique in that they bear 10-20 times more enzyme activity at their surface than similar murine and human cell types. Accordingly, the L-1210 cell offers a useful model cell type to probe the function of such enzymes at the cell surface and to study basic mechanisms of membrane biosynthesis and turnover. In the proposed research, studies on the L-1210 sialytransferase will be extended to accomplish the following specific aims: (1) To determine the cellular distribution of sialic acid and intracellular sialytransferase using the precursor, 3H-N-acetylmannosamine. (2) To examine the relationship between intracellular sialic acid and cell surface sialic acid. (3) To study the effects of plasma membrane utilization (i.e. phagocytosis) and intracellular membrane pools. (4) To evaluate the significance of membrane internalization in the overall metabolic fate of plasma membrane glycoproteins. (5) To partially purify the plasma membrane acceptor molecule of the ectosialytransferase system of L-1210 cells. These studies are all designed to address basic questions regarding the biosynthesis and turnover of plasma membrane in tumor cells, and to assess the possible significance of sialytransferase at the cell surface of L-1210 cells.